In forming high contrast images necessary for Graphic Arts processes by development of silver halide photographic elements, special developers known in the art as "lith" developers are used. The high contrast is achieved by means of the infectious development as described in Journal of the Franklin Institute, vol. 239, 221-230 (1945). These developers exhibit an induction period prior to the development of exposed silver halides, after which the infectious development occurs, which gives rise to the high contrast.
The typical "lith" developer contains only a single developing agent of the dihydroxybenzene type, such as hydroquinone. To enhance the infectious development, "lith" developers contain a low content of alkali sulfite. This low sulfite content renders the developer more prone to aerial oxidation, especially when it is used in combination with processing machines and, more particularly, with Rapid Access type processing machines, where developer degradation is accelerated.
The delay in the start of development caused by the long induction period of hydroquinone developers lengthens the processing time and delays access to the finished material. While the induction period has been eliminated and processing time reduced by using the so called "Rapid Access" developers, which contain both hydroquinone and a superadditive developing agent such as phenidone or metol, these Rapid Access developers are not useful for lithographic purposes because they cannot produce the necessary high contrast. This is because Rapid Access developers have a high sulfite content which prevents infectious development and causes a lower contrast than "lith" developers.
Several alternatives to using a hydroquinone developing agent with a low sulfite content of the "lith" processing system to achieve high contrast development are known in the art. They are the so-called "high contrast" processing systems which use a hydrazine compound, either in the photographic element or in the developing solution, to promote high contrast. The use of hydrazine compounds allows the use of auxiliary developing agents in combination with the dihydroxybenzene developing agent to increase the developing capacity. It also allows the use of relatively high sulfite concentration to protect the developing agents against oxidation, and thereby increasing the developer stability. The high pH level, about 10.5 to 12.8, necessary to obtain the high contrast from the use of hydrazine compounds makes the life of the developing solution relatively short.
Processes which make use of hydrazine are disclosed in U.S. Pat. Nos. 2,419,975; 4,168,977 and 4,224,401. Modifications and improvements to the hydrazine process are disclosed in U.S. Pat. Nos. 2,410,690; 2,419,974; 4,166,742; 4,221,857; 4,237,214; 4,241,164; 4,243,739; 4,272,606; 4,272,614; 4,311,871; 4,323,643; 4,332,878 and 4,337,634 and in Research Disclosure No. 235, Nov. 1983, Item 23510 "Development nucleation by hydrazine and hydrazine derivatives". Despite the improvements which have been made in the hydrazine process, a remaining inconvenience is the relatively low stability of the developer to aerial oxidation, which is a consequence of the high pH required to achieve the desired high contrast.
Contrast promoting agents have been described in U.S. Pat. Nos. 4,172,728 and 4,269,929, in EP 155,690 and in the above cited Research Disclosure which, incorporated in the developing solution, allow the photographic element, including the hydrazine compound, to reach the desired high contrast at a lower pH.
High contrast developing compositions which contain amino compounds and are intended for carrying out development in the presence of a hydrazine compound are also disclosed in U.S. Pat. Nos. 4,668,605 and 4,740,452. In fact, U.S. Pat. No. 4,668,605 describes developing compositions containing a dihydroxybenzene, a p-aminophenol, a sulfite, a contrast promoting amount of an alkanolamine comprising a hydroxyalkyl group of 2 to 10 carbon atoms and a mercapto compound. The developing compositions of U.S. Pat. No. 4,740,452 contain a contrast promoting amount of certain trialkyl amines, monoalkyl-dialkanolamines or dialkylmonoalkanol amines. However, the need to use the contrast promoting agent in a large amount and the volatility and odor-generating characteristics-of amino compounds that are effective in enhancing contrast represent disadvantageous characteristics of the developer solution therein described.
U.S. Pat. Nos. 4,937,160 and 5,190,847 disclose photographic silver halide elements containing an aryl hydrazide having a cationic group for the production of images with ultrahigh contrast. The photographic elements can be developed at relatively low pH, in the range 10.0-11.5 values, by a developing solution including contrast promoting agents such as alkanolamines or secondary aliphatic or aromatic alcohols. Low fog and low tendency to form black spots in unexposed or slightly exposed areas are obtained.
U.S. Pat. No. 4,777,118 discloses a process for forming a high contrast negative photographic image by developing a silver halide photographic element, in the presence of a hydrazine compound as nucleating agent, with an alkali aqueous developing solution which contains a combination of developing agent comprising hydroquinone or substituted hydroquinone and a superadditive developing agent and an antioxidant, wherein the developing solution has a pH lower than about 12 and wherein the silver halide photographic element comprises, an emulsion layer including surface latent image negative type silver halide grains in reactive association with a contrast promoting agent, preferably a diarylcarbinol compound, in a quantity useful to increase contrast. However, in practice, said photographic elements produce photographic images having high contrasts and adequate image quality only when processed by a developing solution at a pH over 11.5. In addition, at lower pH values, in the range from 9.0 to 11.0, the developer solutions are not stable enough. In particular they are very sensitive to atmospheric oxygen, despite their high sulfite content.
U.S. Pat. No. 4,914,003 describes a photographic material containing a generic hydrazine compound as nucleating agent and a specified amine compound as contrast promoting agent to obtain a superhigh contrast negative image when processed by a developing solution having a pH value of 10.5 to 12.0.
Recently, it has been found that well defined photographic elements can be developed in developing solutions having pH below 11.0. For example, U.S. Pat. No. 4,975,354 discloses photographic elements, particularly useful in the field of graphic arts, which are capable of high contrast development, when processed in the presence of a hydrazine compound that functions as a nucleating agent. The elements include certain amino compounds which function as incorporated booster. The characteristics of the compounds contained in the photographic element allow the photographic element to be processed in a developer solution having a pH value in the range of 9 to 10.8.
The photographic materials useful to obtain high-contrast images have the disadvantage of loosing the good sensitometric properties during ageing.
The use of dextran is known in black-and-white X-ray film, see for example Japanese Patent Application 04-125,626, wherein the use of hydrophilic polymers, such as dextran and polyacrylic acid, are introduced in photographic layers to control the water content. This practice also provides a high sharpness material without abrasion defects and with good drying characteristics.
Japanese Patent Application 04-019,648 discloses the replacement of gelatin with dextran to obtain high sensitivity and good tone suitable for reproduction of medical X-ray materials.
U.S. Pat. No. 4,710,456 describes the use of dextran for improving the covering power of developed silver without a deterioration in a) the adhesion between a subbing layer and a silver halide photographic emulsion layer and b) roller marking properties.
U.S. Pat. No. 4,916,049 discloses a photosensitive material which contains a hydrophilic colloid layer containing high molecular weight dextran (more than 100,000) to improve adhesion of sensitive emulsion layer.
U.S. Pat. No. 4,920,032 discloses a photosensitive material which contains dextran and a cationic compound containing a quaternary nitrogen to give the material high sensitivity and high scratch resistance when wet.
U.S. Pat. No. 4,186,010 describes a black-and-white photographic element comprising a compound for increasing covering power selected from the group consisting of dextran and an alkali salt of a low molecular weight maleic anhydride polymer, and a polymeric compound containing recurring units with ketoiminoguanidium groups.
U.S. Pat. No. 5,019,494 describes a silver halide photographic material which comprises a support having thereon a colloid layer which comprises a binder comprising a dextran having a molecular weight of at least 100,000 and an ionic polymer to give improved sensitivity and to be suitable for coating over prolonged periods of time.
U.S. Pat. Nos. 4,975,354; 5,041,355 and 5,126,227 describe a photographic material containing specified classes of hydrazine compounds and of contrast promoting agents useful in graphic arts to obtain high contrasts. The preferred binder disclosed in said patents is gelatin, but other binders such as, for example, dextran, gum arabic, zein, casein, pectin and the like can be used.
It would be desirable to obtain a photographic material providing a very high contrast and good dot quality upon development, in the presence of a hydrazine compound and of a contrast promoting agent, with a conventional Rapid Access type developer solution at a pH value lower than 11.0, said material also having very good ageing properties.